Proximity communication system with wheatstone bridge detector



PROXIMITY COMMUNICATION SYSTEM WITH WHEATSTONE BRIDGE DETECTOR Filed July 31, 1947 Feb. 28, 1950 G w BAUGHMAN 2,499,177

6002 1 01 A/z'eropba aa IN V EN TOR.

$901122? Bacqyhmarz. BY (1 [bodying my invention and shall then .novel features thereof in claims. I I

Patented Feb. 28, last WITH WHEATSTONE BRIDGE DE- TECTOR George W. Baughman, Swissvale, Pa., assignor to The Union Switch &, Signal Company, Swissvale, Pa., a corporation of Pennsylvania 3 Application July 31, 1947, Serial No. 764,949

6 Claims. 1

My invention relates to traflic controlling ap- I paratus, and more particularly to improved proximity detecting apparatus of the electronic. type for the detection of the presence of a vehicle or airplane on the ground in a protected area, such as an airport zone, which apparatus provides means for communication between the vehicle or airplane and a control tower.

My invention is an improvement upon that disclosed in my prior application for Letters Patent of the United States, Serial No. 625,561, filed October 30, 1945, for Railway trafiic controlling apparatus, and comprises a system for enabling the presence of a vehicle or other movable object in a stretch of roadway or in an airportarea to r,

be reliably detected without reliance upon physical contact with the earth, which embodies the closed circuit principle of operation and is thus suitable for use in indicating traffic conditions or for controlling traffic in a manner analogous to the control of railway trafiic by means of track circuits.

The principal object of my invention is the provision in a proximity detecting system of this character of facilities for communication by voice or code with a vehicle or airplane approaching or occupying the protected area or for the control of indication means on the vehicle or airplane from a central ofiice or control tower.

A particular object of the present invention is the provision of means for enabling the detecting system to transmit information to the pilot of an airplane approaching an airfield wherebythe particular zone within which he is to land may be identified and its condition of occupancy indicated.

A further object of my invention is to enable a vehicle or airplane on the ground which has entered the protected area to be communicated with for identifying its location in the event of poor visibility or to enable its further movement to be directed from the control tower.

A feature of my invention is that the com munication means referred to functions in such a manner that the operation of the detecting relay which results when an airplane or vehicle enters an unoccupied protected area is indicated on the airplane or vehicle. p

A furtherfeature of my invention is that the communication facilities are so arranged as to not impair the sensitivity of the'proximlty detecting system.

I shall describe one form of apparatus empolnt out the In the accompanying drawing, the reference character L designates a conductor comprising a loop of insulated wire adjacent to and surround; ing the major portion of a protected area or space A, so disposed that when supplied with alternating current it creates a magnetic field which is subject to a detachable variation in the presence of a vehicle or of an airplane 0n the ground with; in the perimeter of the space A as indicated by the broken line. The loop L is connected through a suitable transformer TI to a Wheatstone bridge W, the input terminals of which are supplied with alternating current of a fixed frequency such, for example, as kilocycles per second, by an e1eetronic oscillator comprising a transformer T4 and an electron tube VI energized over connections to the positive or supply terminal B and the negative or ground terminal C of a suitable local source of current. The output terminals of the bridge are connected through a transformer T2 to the input circuit of a detector tube V2 which tube is energized over connections to terminals B and C of the local source, and has a sensitive relay CF included in its plate circuit. Relay CF is in this manner maintained energized, as long as the bridge is unbalanced. f

To obtain a balance, the bridge arms are so proportioned that when the loop L is energized but unoccupied, its impedance is matched by'a balancing impedance so as to reduce the potential across the output terminals of the bridge substane tially to zero, tube V2 being so biased that this reduces the current through relay CR correg spondingly so that relay CF releases. Since this balanced condition corresponds'to the safety condition of the system, means are provided for making the indication of the baljanced condition of the bridge dependent upon a distinctivemode of energization of relay CF in order to conform with the closed circuit principle of operation which is a necessary requisite for traffic control systems. This is accomplished by the provision of a code transmitter relay CT, of the pendulum type or the like, which is energized fro'm'terminals B and C of the local source and opens its contacts periodically for brief periods at a fixed low rate. Contact a of relay CT, when closed, connects a condenser Cl across the loop terminals, condenser Cl preferably being of such a value as to tune the loop to parallel resonance and thereby unbalance the bridge, the balanced condition'by which relay CF is released being at.- tainable only during the intervals when contact a or relay CT is open.

I It follows that when the loop L is energized and relay CF, decoding apparatus of a conventional type is provided. This comprisesa decoding transformer DT having a primary *winding supplied with impulses of alternately opposite polarity in response to the periodic operation of relay CF, and a secondary winding 'whi'chsupplies alternating current through a decoding-unit DU resonant at the code frequency to a rectifierRl to effect the steady energization of a slow-release detector relay CD, controlling suitable indication or traffic controlling means illustrated typically by the lamps G and R.

This decoding "apparatus is generally similar to that disclosedin Letters'Patent of the United States No. 1,773,472, issued August 19, 1930, to Paul N. Bossart, for Railway traffic controlling apparatus, and a detailed des'cription'thereof is deemed unnecessary.

.It will be readily apparent that relay CD'is normallyheld energized providedthe system is in working order and the loop -L is unoccupied, that relay CD releases when an airplane or ground vehicle enters the protected area A'so as to unbalance the bridge," and is reenergized when the area'is vacated, and further, that relay CD "releases'in the event of loss of energy -"or "the oc- 'currence of a fault in the detecting system.

In accordance with "my invention, the detecting 'system is arranged to provide additional facilities which will now be described.

The oscillator tube VI as shown has the secondary winding of a transformer T3 includedin its cathode circuit which serves, when 'voiceor code frequency current is supplied "to the primary of transformer T3, to periodicallyvary the potential supplied by'transformer' Td'tothe circuit by which tube VI is maintained "inos'cillation and so serves to modulate the high-frequency current supplied'to the loop atthe voiceor code frequency rate. 'Each airplane is provided with aradi'o receiving set as indicated-by the legend "Rec. on the drawing, which "can be -tunedto receive signals of thecarrier frequency supplied toitlie loopand iscapable of detecting the mod ulation'thereof by transformer'T3.

If the loop is unoccupied, relayCD is held energized as already'explained, and byclosing 'a' switch SLthe operator is enabled to connect asuitable reproducer through an amplifierVB to transformer T3, over the front contacts band i: of relays CD and CT, to communicate with the planeby the transmission of voice or code frequency currents generated'bythe reproducer.

'Itfzwillbe noted that the current is modulated :only during the periods when the front contacts *ofrelay CT'are closed, at'which 'timethebri'dg'e "is unbalanced. At'this time loop L is tuned'to parallel resonance by condenser C I, whichpgreatly increases the loop current and thereby'increases "the radiation and the range of the loop as atradio transmitter so as to enable the 'm'odulation'to'be "detected at a greaterdistance;

It is contemplated that the reproducer will be arranged for control by a phonograph record, "magnetic tape, or the "like, to communicate by words. For example, it maybe-arrangedto repe'atedly transmit the number or"otlierideritification of the air strip upon which the plane is to land. It will be readily apparent that such identification will also indicate to the pilot that the particular air strip is unoccupied.

It will be understood that the modulation of the current-supplied to the loop produces side band frequencies which-are the :sum and difference of the carrier and modulation frequencies, having the effect of a change of frequency which might prevent the-bridge from being balanced.

.For-that reason the modulation circuit is opened at contacts band 0 of relay CT each time contact a of that relay opens to place the bridge 'inc'onditionto be balanced. It follows that the accurate balancing of the bridge is not interfered with and' the difficulty of balancing the bridge when supplied with currents of varying frequency is thereby avoided.

"Wh'en'a plane lands within the area A, relay CD releases to indicate the occupancy of the area A-to the'tower operator, and due to I the fact that the reproducer is disconnected bythe opening of the front contacts of-relay CD, its release is also indicated to the pilot of the plane. 1f switch S2 is closed, the closing of the back-contacts-of-relay CD connects a coder or microphone through a suitable amplifier to transformer T3 so that the current supplied to the p may'be modulated for communication with-an airplane or vehicle onthe ground=within the area A.- As long as relay CT is in operationythis communication circuit is intermittently available, but the rate of operation of relay CT is generallyofthe order of three cycles per second or less-and preferably slow-enough to permit a distinctive signal of the Morse code type *to' be transmittedduring each period when relay=CT isreleased, foridentifying the" area A orfor 'directing themovements of an airplane or vehicle therein; By closing 'a switch S3 the operator may hold relay CT-energized and'therebyren'der the circuit available for communication for an extended period, enabling 'a'microphone tobe used for directing traffic within the area A. .It will be'u'nderstood that"each vehicle which is to enter the area may be provided'with a receiver'such as the one indicated on the drawing'by'the legend Rec, responsive to the signals from the coder or microphone.

It will also be understood that since the operation "of relay CD 'is in "accordance with the closed 'circuitprinciple, its contacts may "be employed in safety circuits forinterlocking purposes or for the control of wayside signals for the eontrol of 'tramc entering orapproa'ching the area A in the same manner as track relays are employed for the control of railway. traflic.

Although I have herein-shown and described only one'form of apparatus embodyingmyinvention, it Will be understood that'various changes and modifications may be' made therein within the scope of'the appended claims without departing from the spirit and scope of my invention.

Having thus described my invention, "what I "claim'is:

1. A system for indicating the presence of a movable object'in aprotected area comprising, a conductor arranged'to form aloop surrounding the major portion of said area, meansfforener- "gizing said conductor by alternating carrier currenttocreate a magnetic field .insai'd area, comprising a source of said current and a bridge'having four arms formed by four impedance. elements connected in'thef form of a'square with two input terminals at diagonally opposite corners and two output terminals at the other't'wo corners, said aaoonw bridge having its input terminals connected to the source of said current and having said- -lo'op included in oneof its arms, the other arms of said bridg comprising fired impedances of such value that the bridge isbalanced to current ofthe carrier frequency when said area is unoccupied and is efiectlvely unbalanced when a movable object such as a vehicle or airplaneenters said area, a sensitive relay, means responsive to the' voltage at the output terminals of saidbridge for causing said relay to assume its energized position when the bridge is eiTectively unbalanced and to assume its released position when the bridge is balanced, a mobile receiver, means for modulating the carrier current supplied from said source to transmit communication signals to said receiver when outside of said area, unbalancing means operating periodically between two positions, means controlled by said unbalancing means in one position for effectively unbalancing the bridge by tuning the loop. substantially to parallel resonance to increase the current transmitted to said receiver and to energize said sensitive relay due to the resulting unbalance, means for interrupting the modulation of said current when said unbalancing means assumes its other position to enable the bridge to be balanced to efiect the release of said relay in the event the area is unoccupied, a detector relay, means for causing said detector relay to assume its energized position only in response to the periodic operation of said sensitive relay, and means governed by said detector relay for controlling the character of said modulation whereby signals of one character or another are transmitted to said receiver depending upon whether said detector relay is energized or released.

2. A system for indicating the presence of a movable object in a protected area comprising, a conductor arranged to form a loop surrounding the major portion of said area, means for energizing said conductor by alternating carrier current to create a magnetic field in said area, comprising a source of said current and a bridge having four arms formed by four impedance elements connected in the form of a square with two input terminals at diagonally opposite corners and two output terminals at the other two corners, said bridge having the source of said current connected to its input terminals and having said loop included in one of its arms, the other arms of said bridge comprising fixed impedances of such value that the bridge is substantially balanced to current of the carrier frequency when the area is unoccupied and become effectivel unbalanced when r a movable object such as a vehicle or airplane enters said area, a sensitive relay, means controlled from the output terminals of said bridge for causing said relay to assume its energized position when the bridge is efiectively unbalanced and to assume its released position When the bridge is balanced, means for periodically unbalancing the bridge to cause the relay to periodically assume its energized position when the area is unoccupied, a detector relay, means for selectively energizing said detector relay in response to the periodic operation of said sensitive relay, a mobile receiver responsive to said carrier current, and means controlled by said detector relay for modulating the carrier current supplied to said loop to transmit communication signals to said mobile receiver for indicating the condition of occupancy of said area.

3. A system for indicating the presence of a movable object in a protected area comprising, a

conductor arranged to form a'loop surrounding the major portion of 'said area, means for energizingsaid conductor by alternating carrier cur-. rent to create a magnetic field in said area, comprising a source of said current and a bridge having four arms formed by four impedance elements connected in the formof a. square with two input terminals at diagonally opposite corners and two output terminals at the other two corners, said bridge having the source of said current connected to its input terminals and having said loop included in one of its arms, the other arms of said bridge comprising fixed impedances of such value that the bridge is substantially balanced to current of the carrier frequency when the area is unoccupied and becomes effectively unbalanced when a movable object such as a vehicle or airplane enters said area, a, sensitive relay, means controlled from the output terminals of said bridge for causing said relay to assume its energized position when the bridge is effectively unbalanced and to assume its released position when the bridge is balanced, unbalancin means, a condenser, means controlled by said unbalancing means for periodically connecting said condenser across the terminals of said loop whereby the current is increased due to resonance at the carrier frequency and said sensitive relay is energized due to the resulting unbalance, a detector relay, means for selectively energizing said detector relay in response to the periodic operation of said sensitive relay, a mobile receiver responsive to said carrier current, and means controlled by said detector relay when energized for distinctively modulating the carrier current when said current is increased by said unbalancing means, to effect the transmission of signals from said loop to said mobile receiver for indicating whether said detector relay is energized or released.

4. A system for indicating the presence of a movable object in a protected area of the type described in claim 2, in which the mobile receiver is mounted on a vehicle or airplane adapted to traverse the protected area and in which the modulation of the carrier current supplied to the loop when the detector relay is energized constitutes signals repeated periodically b which the unoccupied condition of said area is indicated to the operator of said vehicle or airplane and the modulation of the carrier current supplied to the loop when the detector relay is released constitutes signals of a different character for directing the movement of said vehicle or airplane within said area.

5. A system for indicating the presence of a movable object in a protected area of the type described in claim 2, in which the mobile receiver is on an airplane approaching said area and the protected area is one of a plurality of similarly equipped areas in difierent landing strips in an airport, and in which the modulation of the carrier current supplied to each loop when the corresponding area is unoccupied constitutes a distinctive signal repeated periodically, said signal identifying to the pilot of said approaching airplane the particular strip upon which he is to land his airplane and also indicating the unoccupied condition of the selected landing strip.

6. A system for indicating the presence of a movable object in a protected area of the type described in claim 2, in which the protected area is in a landing strip in an airport and the mobile receiver is mounted on a vehicle or airplane adapted to traverse said area, and in which the modm latieflofithe carrier current snppIied-1to1thei1onp Number, Name fiafig when such-i area. is. occupied; constitutes; si nals 114539387 Espenschied, 1923 01: directing themovement of saidzvehicleeon air- 137733472 Bbs'sart 1 1930 Plane-F 971 549 WoodwEIIIIII. Aug; 28 1934 GEORGE BAUGHMAN- 5 mom-539* Whitelockr; Dec. 15; 1936 2;I05",930I Reichard; Jan. 18;19'3'8 REFERENCES CITED 2;27Q9,184' Sorensen Apr: 7; 1942 The following references are of record" in the 2355,395 Rubenstein Aug. 8, 1944 me Of this patent: 2,405,501 Halstead ling: 6; 1946 Z;442;a51- Hal'stead June?8=, 194a UNITED STATES PATENTS Number Name: Date 1',446 ,385- Hanson Feb. 20} 1923 

